Detailed information on the flapping motion of insect wings is essential for aerodynamic modeling of insect-flight and the aerodynamic mechanisms being employed in micro-air vehicles (MAV). In recent years, many insects have been studied, but most researches were aimed at the unilateral wing, and detailed study on the flapping deformation of bilateral wings are not yet achieved, especially for the insects with high flapping-frequency, large stroke-amplitude and excellent mobility, such as fly and honeybee. It is obvious that insects can fly freely by the interaction of the bilateral wings, which means that to measure the flapping deformation of bilateral wings will explain the flight mode of insects more clearly.
Untouched observation based on high-speed camera is a general method for measuring the flapping motion of insect wings. Researchers usually use high-speed camera to capture the image-sequences of insect-flight, and calculate the deformation of wings by analyzing those images. According to the difference of system configuration, two kinds of methods can be summarized from the existent technologies. One is single-camera system, which is written in the paper “A Scanning Projected Line Method for Measuring a Beating Bumblebee Wing” presented by L. Zeng, Q. Hao and K. Kawachi. In this method, double laser-sheet sources are used to project multiple parallel light-lines on the wings of a tethered bumblebee and a high-speed camera is used to capture the images, then the deformation of wing is reconstructed by the model of structured-light sensor. Because the stroke and torsion amplitude of bumblebee wing is very large, the surface of wing will be parallel to the projective direction of camera for some moment and no image of wing surface can be formed. This problem is called blind observation region of camera, which results in that only the deformation of a unilateral wing can be measured. Another method is multiple-camera system, which is written in the Paper “The Mechanics of Flight in the Hawkmoth Manduca Sexta” presented by A. P. Willmott, C. P. Ellington. In this method, two cameras are used to observe the flapping motion of a unilateral wing of hawkmoth at two different viewpoints to solve the problem of blind observation region of camera. The main shortcoming of this method is that it is very difficult to simultaneously capture the images of insect-flight by different cameras.
The problems of existent technologies for measuring the flapping motion of insects are analyzed and the configuration of multiple virtual cameras applied in stereo-vision is introduced. FIG. 2 shows a typical stereo vision system with two virtual cameras, from which we can see that one real camera is imaged to two virtual cameras by the reflection of two planar reflectors and a prism with two reflection planes. For a point P in the observation region, two image points are formed on the image plane of camera, which also can be explained that two virtual cameras observe point P at two different viewpoints. According to this configuration, we can compose a geometrical optic component by reflectors to image one real camera to multiple virtual cameras to observe the insect-flight at different viewpoints and avoid the blind observation region of camera. Moreover, the most important is that the asynchronism in image capturing of different cameras dose not exist as all the virtual cameras are imaged from one real camera.
In the configuration of multiple virtual cameras, every two virtual cameras form one stereo sensor. FIG. 3 shows the model of stereo sensor composed of two cameras. For a point P in the observation region, two image points P1 and P2 appear respectively on the image planes of two cameras by two straight projective lines crossing with the image planes, and the spatial coordinate of P can be calculated by the intersection of these two projective lines. Moreover, if the image position of P on image plane 1 is known, its corresponding position on image plane 2 must locate on a straight line, which we call “epipolar line”. Using multiple virtual cameras to observe the flight of insect, we can determine the spatial coordinate of wing-edges by the model of stereo sensor.
Because the flapping-frequency of insect wings is very high, the features on the surface of wings are very blurry in the images captured by the camera and only wing-edges can be extracted to calculate the deformation of insect wings. In order to get more detailed information about the surface of wings, active means is needed to project some features with characteristic geometrical configuration to the insect, such as laser-sheet source with grating for casting multiple parallel light-lines with strong intensity. FIG. 4 shows the theory of reconstruction by grating structured-light sensor, from which we can see that multiple distorted light-lines are formed by laser-sheets crossing with the surface of object to be measured. The spatial coordinate of point P on laser-sheet can be calculated by the intersection of a straight projective line and the plane of laser-sheet. Based on this model, the three-dimensional deformation of wings surface can be reconstructed after the image positions of distorted light-lines are detected.
In addition to the configuration of observation system, effective guiding equipment is necessary for leading insect to fly though the observation region because the excellent mobility and high motion-speed of insect makes it very difficult to capture the images. Many methods have been proposed for different insects according to their biological behaviors, such as food guidance and light guidance. For example, Dudley and Ellington initiated and sustained the flight of bumblebee by ultraviolet light and two rotating drums with alternating black and white stripes covered, which is written in the Paper “Mechanics of Forward Flight in Bumblebees”. In real experiment, we choose honeybee as the research object, which is the representative of insects with high flapping-frequency and large stroke-amplitude. After many experiments for analyzing the habits of honeybee, an optoelectronic guiding equipment is developed to lead the flight of honeybee and trigger the camera to capture the image sequences automatically.